首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
Geology According to the professor, how do geologists interpret the presence of erratics in the tropics?
Geology According to the professor, how do geologists interpret the presence of erratics in the tropics?
admin
2014-09-29
46
问题
Geology
According to the professor, how do geologists interpret the presence of erratics in the tropics?
Geology
Narrator
Listen to part of a lecture in a geology class.
Professor
Um, beginning in the late 1960s, geologists began to uncover some evidence of a rather surprising kind when they looked . . . um ... at various places around the world. What they found out when they examined rocks from about a . . . the period from about 750 million years ago to about 580 million years ago, they found that... it seemed that glaciers covered the entire surface of the Earth—from pole to pole, including the tropics.
Um . . . how did they come to this astonishing conclusion? What was the evidence for this? Especially when glaciers today are found only at the poles ... or in the mountains.
Well, uh .. . basically when glaciers grow and move they leave behind a distinctive deposit consisting of primarily ... of, at least on the top level, of ground up little bits of rock ... almost... they almost look like rocks that have been deposited by streams, if you’ve ever seen those. And that’s caused because, although the glacier is ice, it is actually flowing very slowly and as it moves it grinds the top layer of rock, it breaks off pieces and carries them away. So when you have glaciation you have a distinctive pattern of these pieces of rock which are called "erratics."
Erratics are rocks . . . they’re the stones that are often carried long distances by glaciers.
So, in the 1960s and onward up through the 1990s, we keep finding evidence for glaciation, no matter what the latitude ... even in tropical latitudes. Now, today there are glaciers in the tropics but only at very high elevations. But 750 million years ago, apparently there were glaciers even at sea level in the tropics.
How could this have happened?
Well, first... the growth of glaciers, uh, benefits, if you will, from a kind of a positive feedback loop called the "ice-albedo effect."
With the ice-albedo effect, glaciers—’cause they’re white—reflect light and heat more ... much more than does liquid water... or soil and rock, which are dark and absorb heat. So, the more glaciers there are, the more heat is reflected, so the climate gets cooler, and glaciers grow even more.
However. .. normally, on a global scale, there is a major process that functions to curb the growth of glaciers. And, that process involves carbon dioxide.
Now, we’re all familiar with the notion that carbon dioxide is what we call a "greenhouse gas." The more carbon dioxide there is in the atmosphere, the more heat the atmosphere retains. That’s what a greenhouse gas does. So, the greenhouse-gas effect is kinda the opposite of the albedo effect.
Um ... now as it happens ... when silicate rocks, which is a very common class of rock, when they’re exposed to the air and to normal weathering, they erode. Carbon dioxide is attracted to these eroding rocks and binds to them, forming calcium carbonate.
Calcium carbonate is eventually washed into the ocean where it settles to the bottom. This process, this forming of calcium carbonate, has the effect of sucking the carbon dioxide out of the air and storing it at the bottom of the ocean.
Now, follow me here. The process that’s sucking carbon dioxide out of the air, keeping the greenhouse gas levels low, cannot happen if the rock is covered with ice.
So, while glaciers reflect light and heat... cooling the Earth, they at the same time cover rocks so there’s less calcium carbonate formed .. . which leaves more carbon dioxide in the atmosphere. Higher levels of carbon dioxide keep the atmosphere warm ... which slows the growth of glaciers. So, it’s a balance, and the glacier growth remains pretty much under control.
Now, what happened 750 million years ago to upset that balance? It seems a relatively simple explanation actually ...
750 million years ago ... all the major continents are rocky, bare, and pretty much lined up along the equator; they hadn’t yet moved to where they are today. So, what happened was, perhaps a slight cooling of . . . the very slight and temporary cooling of the Sun—which still happens from time to time—and the Earth starts to cool, the ice starts to spread on the oceans ... starting at the poles.
Now, by the time the ice reaches about two-thirds of the way to the equator, it’s too late.
See .. . because the continents are the last things to be covered by glaciers, they continue weathering . . . the rocks keep eroding and the carbon dioxide levels keep falling ... So, the ice-albedo effect from the glaciers is increasing in strength while the atmosphere continues to lose its ability to retain heat making glacier growth unstoppable. Now you have what’s called a "runaway freeze." And for perhaps as long as 50 million years, possibly with some interludes, the Earth was frozen from pole to pole, like a giant snowball.
选项
A、It indicates that carbon-dioxide levels were once higher there.
B、It is evidence of global glaciation.
C、It indicates that the Earth may cool off at some point in the future.
D、It is evidence that some glaciers originated there.
答案
B
解析
转载请注明原文地址:https://kaotiyun.com/show/JUfO777K
0
托福(TOEFL)
相关试题推荐
Completethetablebelow.WriteNOMORETHANTHREEWORDSORANUMBERforeachanswer.
Completethenotesbelow.WriteNOMORETHANTWOWORDSforeachanswer.MassStrandingsofWhalesandDolphi
ChooseTWOlettersA-F.Accordingtothespeaker,whatareTWOadvantagesofreducingclasssizes?Amoreemploymentforteacher
Listentothedirectionsandmatchtheplacesinquestions11-15totheappropriateplaceamongA-Eonthemap.HealthCentre
Completethesentencesbelow.WriteNOMORETHANTHREEWORDSforeachanswer.BooksofEconomicsarekeptinthe______
Choosethecorrectletter,A,BorC.Theresultsofthequestionnaireshouldbe
Completethenotesbelow.WriteNOMORETHANTHREEWORDSforeachanswer.GIVINGASPEECHReasonsfornervousnessLecturersofte
paintings定位句为“…therewerealsosomeimportantitems,aVictorianrockinghorseformydaughter,somelargeoilpaintingsoriginati
TheseatofFrance’sNorthAmericanholdingsintheeighteenthcenturywasQuebec,andtheFrenchheritage______dominantthere
A、Aneducatedpopulation.B、Alargesupplyofgoodsandservices.C、Alargenumberofpublications.D、Anindustrializedsociety.
随机试题
动脉粥瘤的粥样物质的主要成分是
链球菌在机体组织内能直接扩散的因素是
眶下间隙感染向颅内扩散,并发海绵窦血栓性静脉炎,其扩散途径通常是
管理信息系统是风险监管的内容和要素之一。监管部门对管理信息系统有效性的评判可用()衡量,这些因素受信息需求分析和系统设计影响。
根据《民事诉讼法》规定,在执行过程中,()的,执行终结。
下列各项中,按照有关规定应当征收资源税的有()。
民族区域自治是与中国的国家利益和各民族人民的根本利益相一致的。()
【F1】Imagineeverystudenthasatirelesspersonaltutor,anartificiallyintelligentcompanionthatmagicallyknowseverything,
Themainpurposeofthistextisto______.Atwhichofthefollowingtimeswillthebusleavethemainhall?
Choosethecorrectletter,A,BorC.
最新回复
(
0
)